1. Field of the Invention
The present invention relates to an improvement of measurement accuracy in the case of determining a distance from an object according to a Time of Flight Method, and particularly to a distance measuring apparatus and method using a pulsed electromagnetic wave, by which a precise distance measurement can be performed with a high-speed response even when the distance is small.
2. Disclosure of the Prior Art
A method of determining a distance from an object according to a time period between a projection time, at which a pulsed electromagnetic wave is projected to the object, and a reflection-wave reception time, at which the electromagnetic wave reflected from the object is received is known as a Time of Flight Method.
For example, U.S. Pat. No. 5,054,911 discloses a light wave distance measuring instrument of the pulse type. According to this instrument, a part of the light wave projected to an object is used as a reference light wave, and allowed to pass alone an optical path having a predetermined length without interacting with the object. A reflection wave obtained when the light wave is reflected off the object and the reference wave are received by a single receiver. A distance between the object and the instrument is determined according to a time difference between a reference-wave reception time, at which the reference wave is received by the receiver, and a reflection-wave reception time, at which the reflection wave is received by the same receiver.
By the way, as shown in FIG. 24, when the distance between the instrument and the object is small, the reference wave xe2x80x9cSxe2x80x9d may be positioned closely adjacent to the reflection wave xe2x80x9cRxe2x80x9d on the time axis. In this case, it becomes difficult to clearly separate the reference wave xe2x80x9cSxe2x80x9d and the reflection wave xe2x80x9cRxe2x80x9d from each other, so that there is a problem in that the measurement accuracy is lowered. To avoid such a problem, it was proposed to introduce an optical fiber having a predetermined length into an optical path for the reflection wave, and delay the reception of the reflection-wave by a delay time xe2x80x9cTdxe2x80x9d such that the reflection wave xe2x80x9cRxe2x80x9d is spaced away from the reference wave xe2x80x9cSxe2x80x9d on the time axis, as shown by the dotted line in FIG. 24. Thereby, the distance can be determined according to a time difference xe2x80x9cTS-Rxe2x80x9d between the receptions of the reflection wave and the reference wave and the delay time xe2x80x9cTd.xe2x80x9d
However, the reflectivity of the object changes according to the color of the object surface, i.e., whether the surface color is black or white. In addition, the reflectivity changes according to the surface condition of the object, i.e., whether the object surface is a mirror reflection surface or a diffuse reflection surface. Therefore, when the reflectivity is small, there is a concern that a considerable loss of light can result from allowing the reflection wave to pass through the optical fiber. Needless to say, as the length of the optical fiber increases, the problem of light loss becomes more serious. In other words, it reduces the S/N ratio. For example, to prevent this S/N ratio deterioration, the gain of an amplifier or an amount of the light wave projected to the object can be increased. However, these solutions to the S/N deterioration problem lead to an increase in cost of the distance measuring instrument. In addition, there is another problem in that the distance measuring instrument becomes a huge and complex structure.
Therefore, in consideration of the above, a primary object of the present invention is to provide a distance measuring apparatus using a pulsed electromagnetic wave, by which a precise distance measurement can be performed with a high-speed response even when a distance between the apparatus and an object is small.
That is, the distance measuring apparatus includes:
a single projector for projecting the electromagnetic wave to an object;
a branch means placed between the projector and the object to obtain a reference wave branched from the electromagnetic wave;
a single receiver for receiving the reference wave and a reflection wave obtained when the electromagnetic wave is reflected off the object;
a delay means introduced into an optical path for the reference wave extending from the branch means to the receiver without interacting with the object to provide a delay time for delaying a reference-wave reception time, at which the reference wave is received by the receiver, such that a first time period between the reference-wave reception time and an electromagnetic-wave projection time, at which the electromagnetic wave is projected from the projector, is longer than a second time period between the projection time and a reflection-wave reception time, at which the reflection wave is received by the receiver; and
a processor for calculating a time difference between the reference-wave reception time and the reflection-wave reception time from outputs of the receiver, and determining a distance between the distance measuring apparatus and the object according to the time difference and the delay time.
According to the distance measuring apparatus of the present invention, since the reference-wave reception time is delayed than the reflection-wave reception time by the delay time, it is possible to prevent the occurrence of an inconvenience that the reference wave is overlapped with the reflection wave, as shown in FIG. 24, so that the distance can not be measured with accuracy. In particular, since maintaining a high strength of the reference wave is relatively easy, a light loss of the reference wave caused by allowing the reference wave to pass through the delay means such as an optical fiber to delay the reference wave reception time does not wield a large influence over the accuracy of measuring the distance.
When the electromagnetic wave is light, it is preferred that the delay means is provided by an optical fiber having a predetermined length or a plurality of mirrors arranged so as to prolong the optical path for the reference wave. In the case of using the optical fiber, it is possible to obtain a desired delay time according to the length of the optical fiber. In the case of using the mirrors, it is possible to stably provide the delay time even in the presence of disturbance factors such as changes in ambient temperature
In addition, it is preferred that the delay means includes a reference-wave receiving device used only to receive the reference wave from the branch means; a delay circuit for delaying an output signal provided from the reference-wave receiving device; and a reference-wave emitting device for providing an output of the delay circuit to the receiver. This embodiment is effective to downsize the distance measuring apparatus of the present invention.
It is preferred that the delay means includes a delay-time adjuster for changing the delay time. In this case, since an adequate delay time can be selected in accordance with the distance to be measured, it is possible to accurately determine the distance with an improved response speed.
It is also preferred that the distance measuring apparatus further includes a temperature compensating unit for compensating for fluctuations of the delay time, which is caused by changes in ambient temperature. In this case, it is possible to stably maintain the measurement accuracy without the influence of ambient temperature.
When the electromagnetic wave is light, it is preferred that the distance measuring apparatus further includes a light-amount adjuster for adjusting a light amount of at least one of the reflection wave and the reference wave received by the receiver. Since adjusting the light amount of the reflection wave and/or the reference wave is effective to reduce noise components, it is possible to improve the S/N ratio and to more accurately determine the distance.
It is preferred that the processor allows the projector to make a plurality of projections of the electromagnetic wave to the object, and calculates an average time difference between the reference-wave reception time and the reflection-wave reception time from the outputs of the receiver provided for each of the plurality of projections of the electromagnetic wave to determine the distance according to the average time difference and the delay time. This embodiment is particularly useful when it is needed to determine the distance with high accuracy. In addition, this embodiment is preferably used when a preliminary measurement of roughly determining the distance is performed by using a provisionally-determined delay time, and then the delay time used in an actual measurement of precisely measuring the distance is adjusted according to results of the preliminary measurement.
It is preferred that the delay means provides the delay time longer than a time period required to allow the electromagnetic wave to pass through double a maximum measurable distance of the distance measuring apparatus. When the distance to be measured is smaller than the maximum measurable distance, it is possible to certainly separate the reflection wave from the reference wave.
A further object of the present invention is to provide a distance measuring method using a pulsed electromagnetic wave, by which the same advantages described above can be achieved. That is method comprises the steps of:
projecting the electromagnetic wave to an object from a single projector;
receiving, by a receiver, a reflection wave obtained when the electromagnetic wave is reflected off the object;
receiving, by the same receiver, a reference wave branched from the electromagnetic wave before the electromagnetic wave reaches the object; and
determining a distance from the object according to outputs of the receiver;
wherein the method is characterized in that a reference-wave reception time, at which the reference wave is received by the receiver, is delayed by a delay time, so that a first time period between an electromagnetic-wave projection time, at which the electromagnetic wave is emitted from the projector, and the reference-wave reception time is longer than a second time period between the projection time and a reflection-wave reception time, at which the reflection wave is received by the receiver, and the distance from the object is determined according to the delay time and a time difference between the reference-wave reception time and the reflection-wave reception time.
In the above method, it is preferred that the delay time is changed according to a large or small distance from the object. By selecting an adequate delay time, it is possible to efficiently perform the distance measurement.
In addition, it is preferred that a preliminary measurement of roughly determining the distance from the object is performed by using a provisionally-determined delay time, and then the delay time used in an actual measurement of precisely measuring the distance is adjusted according to results of the preliminary measurement. Since the results of the preliminary measurement are useful to select the adequate delay time, it is possible to smoothly proceed the actual measurement with high accuracy. In particular, when the electromagnetic wave is projected to the object plural times in the actual measurement, and an average time difference between the reference-wave reception time and the reflection-wave reception time is calculated from the outputs of the receiver provided every projection of the electromagnetic wave, so that the distance is determined according to the average time difference and the delay time, it is recommend performing the preliminary measurement.
Moreover, it is preferred that a preliminary measurement of roughly determining the distance from the object is performed by using a provisionally-determined delay time, and then a light amount of at least one of the reflection wave and the reference wave received by the receiver in an actual measurement of precisely measuring the distance is adjusted according to results of the preliminary measurement. By selecting the adequate light amount of the reflection wave and/or the reference wave, it is possible to determine the distance with an improved SIN ratio.
These and still other objects and advantages of the present invention will become more apparent from preferred embodiments of the invention explained below, referring to the attached drawings.